Ships are 'she' not 'it'......She docks at the Marittima pier - about 10mins walk from Piazzale Roma. You will be able to board from around 1pm. Suggest you use the europeforvisitors site for useful info and maps.
We did just this 2007, there was then a complimentary shuttle bus from Piazzale roma to the ship provided by Royal Caribbean, suggest you check to see if this is still available, when your tickets arrive you will be given a boarding time ours was 2.00pm, please don't expect a shuttle bus at the end of your cruise, we were told there was one, but there wasn't, there is a handy left luggage store at the Piazzale Roma for your return have a great time, we di.
We did this trip in 2006 and the Splendour docked at Bacino Statione Marittima (see internet for map) along with all of the other big cruise liners. Under no circumstances try to walk it. There's a water taxi service from the moorings at the end of St Mark's Square and it's a great way to kick off your trip. The Piazzale Roma bus station is itself a good hike from the city centre so I'd recommend that you don't stress yourself out too much by dragging your cases there. Once on the move get yourself as high up on the port side of the ship as you can and soak up the amazing views as she sails down the canal past St Marks and the Bridge of Sighs. I've never forget the experience. I think I forgot to breathe for 10 minutes!